This invention relates to portable impact tools, and, more particularly, to a hammer construction which includes a vibration damping device.
The invention is specifically concerned with a forged carpenter's claw hammer of the so-called "indestructible" type wherein the striking head is formed integrally with a steel shank, the latter constituting a part of the hammer handle.
An indestructible hammer offers a few advantages over a conventional claw hammer of the hickory handle type, but is also possessed of numerous limitations. The advantages which make it popular are increased strength and a permanent union between the hammer head and the shank. In the case of a wooden handle type claw hammer having an impact head for nail-driving purposes and integral claws for nail-pulling or removing purposes, the hammer will ordinarily withstand even the roughest usage when put to the use for which it is intended. However, when it is put to unintended uses, as, for example, wrecking, will frequently be subject to handle breakage or looseness in the joint between the handle and the impact head. This is particularly true after the wooden handle has dried out, as it invariably will, in time. Such is not the case with an indestructible claw hammer having an integral steel shank. These factors are the reasons why such a hammer has met with appreciable success on the market.
On the other hand, indestructible hammers are possessed of numerous limitations, principal among which are: (1) lack of resiliency which renders it awkward in the hand of an experienced carpenter or workman, and (2) the tendency for impact to set up undesired vibrations.
Initially, indestructible type hammers were invariably in the form of a solid steel unit comprising a head, a pair of claws, and a shank. With such a hammer, the force of the impact is carried directly from the head into the shank where it is felt by the hand of the user. At the same time, a secondary and slightly out-of-phase impact is applied to the hand of the user by reason of the initial shock traveling across the head and into the claws which are caused to vibrate and send a secondary impact back into the shank following closely the initial and somewhat stronger impact force. Thus, the solid steel construction of an indestructible claw hammer does not afford significant shock-absorbing resiliency.
Such vibratory effects are not only annoying to the user of the hammer, but they weaken the hammer structurally so that, in time, cleavage or fracture takes place, usually in the vicinity of one or both of the claws. Cleavage has been known to take place directly across the base of a claw, not at a time when the claw is put to use in extracting a nail, but at a time when the claw portion of the hammer is not in use, the cleavage being complete and in the form of a clean fracture across the claw with the claw falling off or separating from the impact head.
In an effort to minimize such undesired vibrations, it is known to redistribute the metal of the impact head by forming therein a relatively deep rectangular socket, the socket extending crosswise of the head and in axial alignment with the shank and serving, in a measure, to divide the impact nose of the hammer head from the claw portion. The four side walls of the socket are relatively thin and much of the shock of impact is dissipated in these side walls. Furthermore, the socket is filled with a vibration dampening substance which further inhibits claw vibration. Such a hammer constitutes the subject matter of U.S. Pat. No. 2,884,969, granted on May 5, 1959 to Clarence M. Lay and entitled "Hammer Construction With Shock Absorbing Means." Reference to this patent reveals the fact that the provision of such a socket in the head of an indestructible type hammer affords advantages other than that of its vibration damping effect which are not, however, particularly relevant to the present invention which is concerned primarily with vibration damping.
A carpenter's claw hammer with vibration damping means is disclosed in U.S. Pat. No. 3,208,724 to Howard A. Vaughan. In order to attain a desired vibration damping effect, the disclosed hammer places ribs on the hammer head, the vibration damping ribs being provided on the inside surface of each claw and running from the base of the claw well into the medial region thereof. While such ribs may have an incidental function of strengthening or rigidifying the claws, they primarily function to inhibit claw vibration. The rectangular sockets in the medial body portion of the hammer is described as being preferably filled with a vibration damping substance, such as a suitable thermoplastic or thermosetting resin or, alternatively, may be filled with a wooden plug.
Rubber sleeves are interposed between the hammer shank and the head in U.S. Pat. No. 2,850,331 to John J. Curry for a handle connection for percussive tool. The resilient sleeve is provided to isolate the handle from vibrations set up in the tool during impact.
In U.S. Pat. No. 2,067,751 to Raymond E. Beegle for a securing means for tool handles and U.S. Pat. No. 2,917,349 to Charles Saylor for a tool handle connection with damp resilient bond, one or more layers or washers of a rubber-like material are provided between the head member and the handle portion to provide deformable shock-absorbent elements for damping vibrations of a head member so as to preclude production of destructive stresses therein.
While the foregoing hammer designs incorporate elements for damping or lessening vibrations following impact, the resilient elements normally interface between the hammer head and the handle. This, of course, means that all of the forces are transmitted through these resilient elements and, in time, these elements may deteriorate, losing their effectiveness and may create a hazardous joint between the handle and the hammer head. In the Vaughan Pat. No. 3,208,724, special ribs are used, and the resins or wooden plug filling the socket in the head become substantially integral with the head and form a single resonant system.